Low cost motor design for rare-earth-magnet loudspeakers

ABSTRACT

A loudspeaker magnetic motor utilizes a voice coil with two or more wire coils that are connected in parallel and that are layered on top of one another. The motor utilizes, as a magnetic field source, a permanent magnet and, more particularly, a permanent magnet that includes a rare earth metal such as a neodymium boron iron magnet.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. patentapplication Ser. No. 60/108,338, filed Nov. 13, 1998. This applicationis also a continuation of U.S. patent application Ser. No. 09/439,416,filed Nov. 13, 1999.

BACKGROUND OF THE INVENTION

The invention relates to loudspeakers and to low-cost magnetic motorsfor use in loudspeakers. The invention has application, among otherplaces, in cell phones, pagers, MP3 players, and other appliances whereweight and size are factors.

A large percentage of loudspeakers are electrodynamic speakers. Suchspeakers employ a magnetic driver to produce movement of a diaphragm(typically cone or dome-shaped) which, in turn, causes sound. A typicalloudspeaker includes a fixed magnet and voice coil. The magnet may bemounted to the rear of the frame behind the diaphragm. A magnetic“circuit” may be utilized to focus and, therefore, intensify themagnetic field in a region referred to as the “air gap”. The voice coilis disposed adjacent the magnet and, preferably, within the air gap. Thecoil typically wire formed about a cylindrical support or “former”which, itself, is attached to the diaphragm.

In operation, electrical audio signals from an amplifier are applied tothe voice coil producing a varying electromagnetic field around thecoil. The electromagnetic field interacts with the magnetic fieldproduced by the magnet. The magnet is securely fixed to the frame andthe voice coil is movable, so the voice coil moves as the two fieldsinteract. Because the voice coil is coupled to the diaphragm via thesupport, its movement causes the diaphragm to vibrate. The vibration ofthe diaphragm causes air around the speaker to pressurize anddepressurize producing sound waves in the air.

The high energy density of rare earth materials such as neodymium boroniron is attractive for creating and miniaturizing shielded loudspeakermagnets. The magnet rings or discs can best be installed as cores on theinside of the transducers voice coil for easy manufacturing. The maximumstorable and extractable energy is then limited by the voice coildiameter and can be increased only by the height of the neodymium slug.

An object of this invention is to provide improved loudspeakers and,more particularly, improved magnetic motors for loudspeakers.

A further object of the invention is to provide such motors that utilizerare earth magnets.

A still further object of the invention is to provide such motors aspermit construction of lower impedance, higher B×L neodymium motors fordriving loudspeakers.

Yet a still further object is to provide such motors as eliminate theneed for multiple magnets and expensive edge winding and offers greaterfreedom in amplifier matching for best overall system value.

Still yet further objects of the invention are to provide such motors aspermit the construction of low voltage sound systems for portabletalking appliances like cell phones, note book and palm size computers,pagers, and other interactive wireless appliances.

SUMMARY OF THE INVENTION

The foregoing objects are attained by the invention which provides, inone aspect, a loudspeaker magnetic motor that utilizes a voice coil withtwo or more wire coils that are connected in parallel and that arelayered on top of one another.

Further aspects of the invention provide motors as described above inwhich the coils are formed from wires that have round cross-sections.

Still further aspects of the invention provide motors as described abovein which a first coil is disposed about a voice coil former and in whicha second coil is disposed about the first coil.

The invention provides, in other aspects, a motor as described abovewhich includes, as a magnetic field source, a permanent magnet and, moreparticularly, a permanent magnet that includes a rare earth metal.Related aspects of the invention provide a motor as described above inwhich the magnetic field source comprises neodymium. One such source isa neodymium boron iron magnet.

Another aspect of the invention provides a motor as described above inwhich the permanent magnet is “coin shaped” or, more particularly, has acylindrical cross-section.

Still other aspects of the invention provide a loudspeaker that includesa magnetic motor as described above.

These and other aspects of the invention are evident in the drawings andin the description that follows.

Loudspeaker magnetic motors as provided by the invention feature severaladvantages over the prior art. They provide a low cost, practical methodfor maximizing the available force F=B×L×I from any “thick” rare earthmagnet motor, i.e., one with a permanent magnet with an operating pointB/H>=2.5. This leads to an improved cost performance ratio by permittingconstruction of lower impedance, higher B×L neodymium motors for drivingloudspeakers. This also eliminates the need for multiple magnets andexpensive edge winding and offers greater freedom in amplifier matchingfor best overall system value.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be attained byreference to the drawings, in which:

FIG. 1 is a graph showing relationships between flux density (B),coercive force (H) and operating points for various magnetic materialsand configurations;

FIG. 2A shows a cross section of conventional magnetic motor; FIG. 2Bdetails a portion of the drawing shown in FIG. 2A;

FIG. 3A shows a cross section of magnetic motor using edge winding; FIG.3B details a portion of the drawing shown in FIG. 3A;

FIG. 4A shows a cross section of neodymium boron iron magnetic motorusing a winding according to the invention; FIG. 48 details a portion ofthe drawing shown in FIG. 4A; and

FIG. 5 is a graph showing increased bass output using same coil andmagnet in a 4-layer versus a “tandem” configuration after normalizingcurves at 500 Hz.

FIG. 6 is a graph showing impedance vs frequency for a magnetic motoraccording to the invention vs that of conventional motor, e.g., of thetype shown in FIG. 2; and

FIG. 7 shows a loudspeaker according to the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The driving force available to a speaker is B×L×I, where B is the fluxdensity, L the length of coil wire and I the current through the coilwire. For a fixed magnet diameter and gap, the height d and thus themagnetic operating point B/H are rapidly reached where the flux densityB in the gap increases very little while the magnet cost increases asits height increases. The full energy product B*H can only be realizedfor B/H=1. See FIG. 1.

FIGS. 2A and 2B show cross sections of conventional magnetic motor 10.The illustrated motor includes a permanent magnet 12 and a magnetic“circuit” comprising top and bottom plates 14, 16. The plates focus thefield of magnet 12 in a gap 18, which is shown in greater detail in FIG.2B. A voice coil 20 is formed about support (or former) 22. Theillustrated coil comprises two layers of round wire, i.e., wire having around cross-section.

The motor 10 of FIGS. 2A-2B is best suited to “low” magneticoperating-point (B/H) systems with d/w>=2.5, where d is the height ofthe magnet and w is the width of the gap. A taller magnet with B/H>=2.5improves mainly the temperature stability of the system.

One way to increase the drive force of a magnetic motor of the typeshown in FIGS. 2A-2B is to utilize stacked magnets of opposing polarity.This can be costly, though effective.

Another way is to use and “edge winding” configuration of the type shownin FIGS. 3A-3B. Here, the active coil wire length L is increased bywinding a “flat wire” 24A (i.e., a wire of flattened cross-section)about the support former. This configuration is particularly useful whenflux density B itself cannot be improved and, hence, only an increase ofwire length L (or current I) can increase the extractable force.

Unfortunately, the process of flat wire coil winding is tedious and tooslow for low cost, high volume production. In practice, edge-windingalso leads to either heavy or high resistance coils. The coil massbecomes prohibitive if a low resistance is to be maintained or theresistance becomes impracticably high thus reducing the current I.

Another drawback is unfilled gap space needed to clear the return wire24B, which occupies a portion of the gap 18 and, hence, preventsextraction of energy that might otherwise be attained from the magneticfield within gap.

FIGS. 4A-4B depict a magnetic motor according to one practice of theinvention. The motor includes a magnet 12′ that preferably comprises arare earth metal and, more preferably, neodymium. Still, morepreferably, it is a neodymium boron iron magnet. Top and bottom plates14, 16 are comprised of materials of the type conventionally used inconnection with such magnets 12′.

Voice coil 20′ comprises two or more windings of wire or other conductorof the type conventionally used in rare earth magnetic motors. Unlikethe conventional configurations (e.g., of the type shown in FIGS.2A-2B), the multiple windings of coil 20′ are connected in parallel.Thus, a first winding is disposed about the cylindrical former 22, asecond winding is disposed about the first, a third winding about thesecond, and so forth. The windings are connected in parallel to oneanother.

A motor according to the invention emulates the edge-woundconfiguration, without the latter's inherent disadvantages. Such windingmultiplies the number of turns L for a given gap length just like anormal round wire coil. The stacked coil sections are then connected inparallel.

In a configuration with multiple windings, for a given applied voltage,the current I increases four-fold compared a conventional two-layer coil(e.g., as shown in FIG. 2) with the same number of turns. The resistanceis one fourth of that of the normal coil and the effective number

of turns L is cut in half. However, the number of turns L for a givencoil height is SQRT(2) times greater than a single-wire coil of the sameresistance and height. Coil thickness of the tandem coil is SQRT(2)times that of a single wire coil of equal area.

For a given flux density B, the B×L×I-product is therefore SQRT(2) timeslarger than a single wire coil of equal area while the mass isapproximately the same.

By using a neodymium boron magnet, the motor of FIGS. 4A-4B permitincreasing the gap width without suffering the loss of flux densityassociated with ferrite magnets when widening the magnet gap.Furthermore they enables powerful magnet designs where a “thick”neodymium magnet can be on the inside of the voice coil and still offera high level of extractable energy. Benefiting applications are handsfree cell phones, pagers, MP3 players, and other new interactive talkinginter net appliances where weight and size are crucial to the productacceptance.

FIG. 5 is a graph shows increased bass output using same coil and magnetin a 4-layer versus a “tandem” configuration after normalizing curves at500 Hz. FIG. 6 is a graph showing impedance vs frequency for a magneticmotor according to the invention vs that of conventional motor, e.g., ofthe type shown in FIG. 2.

Motors according to the invention fulfill the following significantbenefits:

1. Increase of 33% in B×L product while maintaining same moving mass(See FIG. 5);

2. Low drive impedance for improved power intake in low supply voltageapplications (See FIG. 6, yellow curve);

3. Reduced inductance compared to normal multi-layer coil also improveshigh frequency response (See FIG. 6);

4. Low cost construction and manufacturing;

5. Maintains the temperature stability of a high magnetic operatingpoint;

6. Enable four- and six-layer coil construction without undue massincrease;

7. Better utilization of all metal and magnetic materials.

FIG. 7 shows a loudspeaker according to the invention. The speaker is ofconventional operation and construction, except insofar as it includes amagnetic motor of the type shown in FIGS. 4A-4B and described above.

Described above is a improved magnetic motor and loudspeaker accordingto the invention. It will be appreciated that the embodiment shown inthe drawings and described above are merely examples of the inventionand that other motors and loudspeakers, incorporating the teachingshereof fall within the scope of the invention, of which I claim:

1. A loudspeaker magnetic motor comprising a voice coil the voice coil comprising two or more wire coils, the wire coils being connected in parallel and being layered on top of one another.
 2. A loudspeaker magnetic motor according to claim 1, wherein at least one of the coils comprises a conductor having a round cross-section.
 3. A loudspeaker magnetic motor according to claim 2, wherein the coils comprise wires having round cross-sections.
 4. A loudspeaker magnetic motor according to claim 2, in which a first wire coil is disposed about a support, and a second wire coil is disposed about the first coil.
 5. A loudspeaker magnetic motor according to claim 1, comprising a magnetic field source.
 7. A loudspeaker magnetic motor according to claim 5, wherein the magnetic field source is a permanent magnet.
 8. A loudspeaker magnetic motor according to claim 7, wherein the magnetic field source comprises a rare earth metal.
 9. A loudspeaker magnetic motor according to claim 8, wherein the magnetic field source comprises neodymium.
 10. A loudspeaker magnetic motor according to claim 9, wherein the magnetic field source comprises a neodymium boron iron magnet.
 11. A loudspeaker magnetic motor according to claim 10, wherein the neodymium boron iron magnet has a cylindrical cross-section.
 12. A loudspeaker comprising a voice coil the voice coil comprising two or more wire coils, the wire coils being connected in in parallel and being layered on top of one another.
 13. A loudspeaker according to claim 12, wherein at least one of the coils comprises a conductor having a round cross-section.
 14. A loudspeaker according to claim 13, wherein the coils comprise wires having round cross-sections.
 15. A loudspeaker according to claim 13, in which a first wire coil is disposed about a support, and a second wire coil is disposed about the first coil.
 16. A loudspeaker according to claim 12, comprising a magnetic field source.
 18. A loudspeaker according to claim 16, wherein the magnetic field source is a permanent magnet.
 19. A loudspeaker according to claim 18, wherein the magnetic field source comprises a rare earth metal.
 20. A loudspeaker according to claim 19, wherein the magnetic field source comprises neodymium.
 21. A loudspeaker according to claim 20, wherein the magnetic field source comprises a neodymium boron iron magnet.
 22. A loudspeaker according to claim 21, wherein the neodymium boron iron magnet has a cylindrical cross-section. 